Fuel cell power plants are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is known that product water generated by fuel cells of the plant is often removed through porous water transport plates into a coolant system, as well as through evaporation and/or entrainment in a cathode exhaust stream. Such a coolant system is also utilized to provide water through the porous water transport plates to humidify fuel and oxidant reactant streams to thereby minimize drying out of proton exchange membrane (“PEM”) electrolytes. Porous water transport plates when filled with a liquid also provide gaseous seals to prohibit mixing of the reactant oxidant and fuel streams. Fuel cell power plants having porous water transport plates, however, give rise to water management difficulties related to freezing of water when the plant is operated or shut down in sub-freezing conditions.
An alternative fuel cell power plant is known that utilizes passive water management and reduced free water volume, wherein the coolant system includes a sealed cooler plate so that traditional antifreeze solutions may be utilized to cool fuel cells of the plant. Because the cooler plate is sealed, the antifreeze solution cannot pass out of the plate to poison fuel cell catalysts. As described in commonly owned U.S. patent application Ser. No. 10/036,181, now U.S. Pat. No. 6,794,077 B2, entitled “Passive Water Management Fuel Cell”, fuel cell product water may be removed from an operating fuel cell by water management flow fields defined adjacent to anode and cathode reactant flow fields. Such water management flow fields also serve to provide water for humidification of the reactant streams to prohibit drying out of the PEM electrolyte. The water flow fields also provide gaseous seals between cells of a well-known fuel cell stack assembly in the event a sealed cooler plate or solid separator plate is not disposed between each cell of the assembly. It is important that the water management flow fields not dry out, so that reactant streams may not pass out of fuel or oxidant flow fields into the water management flow fields, so that reactant streams do not cross-over the water flow fields to mix together, and so that water is not evaporated out of a PEM electrolyte to humanity dry reactant streams thereby degrading the electrolyte performance. The water management flow fields may be defined as channels or pores within porous layers.
Fuel and oxidant reactant streams passing into a fuel cell may tend to evaporate water out of water management flow fields and the PEM electrolyte adjacent the streams depending upon various factors, including relative humidity of the streams, availability of water, and temperatures of the water management flow fields near the reactant streams, etc. For example, an oxidant stream having a low relative humidity with respect to a temperature of an oxidant inlet of the cell may not dry out a cathode water management flow field because product water being generated by the fuel cell readily replaces water evaporated out of the field into the oxidant stream. However, a fuel stream having a low relative humidity with respect to a temperature of a fuel inlet of the cell may dry out an anode water management flow field, leading to movement of the fuel stream into the water management flow field, to evaporation of water from the PEM electrolyte, and to cross-over of the reactant streams through the anode water management flow field.
Consequently, there is a need for a passive water management system for a fuel cell power plant that provides for humidification of reactant streams, removal of fuel cell product water, minimal free water volume, and that minimizes a risk of drying out of water management flow fields of the plant.